The invention relates to rewriteable optical recording elements of the phase-change type.
Optical disks utilizing focused laser light to record and retrieve information have gained increasing commercial importance in recent years. The success of these products are due to many factors: high storage capacity, ease of replication, robustness, removability, etc., but the availability of extremely low cost rewriteable phase-change optical recording element is at least as important as any of the other factors.
There are basically three types of optical disks: (1) The read only type (ROM) has data fabricated on the disks as part of the manufacturing process. End users can only read the data from the disks but are not able to record on them; (2) The write-once type (WORM) allows end users to record data on the disks but does not allow them to change the data once written. (3) The rewriteable type (RW) allows data to be written and re-written on the disk many times. Although the ROM type seems limited in its capability, it has been commercially the most successful of the three. The main reason is that it allows huge amounts of data to be replicated and distributed at very low cost. It is currently the dominant method for distributing music, video, data, etc. The successful ROM products include CD-Audio, CD-ROM, DVD-Video, and DVD-ROM, etc. Recently the WORM type media CD-R has become very successful also. Its compatibility with the ROM disks and its low cost are generally considered the main reasons for its success. The RW type, on the other hand, is not as successful at the moment, despite the fact that it is the more capable and flexible. There may be many factors for its lack of success, the high cost of media is considered one of the key reason. There is therefore strong incentive to reduce the cost of the media.
There are currently two main types of RW media. The magneto-optic type that uses the change of magnetization in the recording layer to store data, and the phase-change type that uses the transition between the amorphous state and the crystalline state of the material to store data. Each type has its strengths and weaknesses. The current invention deals with the phase-change type.
Many erasable phase-change materials have been used. Examples of the materials include Gexe2x80x94Te, Gexe2x80x94Texe2x80x94Sn, Sbxe2x80x94Te, Sbxe2x80x94Gexe2x80x94Te, Inxe2x80x94Sbxe2x80x94Te, Sbxe2x80x94Texe2x80x94In, and Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te. Although this list contains many material systems with different properties, the basic recording element layer structure is the same and so is the read/write operation.
Rewriteable elements of the phase-change type are usually constructed of multi-layer structures. In addition to the phase-change layer, dielectric and reflector layers are also used to control the optical and thermal properties.
When phase-change materials are used for optical recording, a focused laser light is used to switch the material between the amorphous state and the crystalline state. During the conventional recording process, the laser light is pulsed between three power levels. The higher power level melts the material, the intermediate power level heats the material to just below the melting point and above the crystallization temperature, and the lower level is used to control the heating and cooling of the material. The phase-change material, the multi-layer structure of the disk, and the laser pulse sequence are all carefully designed to ensure that the material melted by the high power laser pulse is cooled quickly to quench in the amorphous state. The same design allows the intermediate power pulses to heat the material to just below the melting point and crystallize the material. Thus by alternating the write laser pulses between the power levels one can produce alternate crystalline and amorphous regions on the disk, and this is how information is stored.
As deposited, the recording layer is usually in the amorphous state. In the amorphous state, the reflectance of the recording element is usually too low to allow reliable focusing and tracking for the read/write operation. To solve this problem, a laser initialization step is generally used as part of the manufacturing process. As is conventionally done, a relatively large laser spot that covers several recording tracks is scanned over essentially the entire surface of the recordable area of the recording element. The power of the laser spot and the speed of scanning is adjusted such that to cause crystallization of the recording material. The crystallization of the recording layer increases the reflectance of the recording element and ensures reliable focusing and tracking.
As is practiced today, however, the initialization step is rather slow and is often the rate limiting step of the media manufacturing process. In fact several pieces of initialization equipment are usually used in a production line to match up with the throughput of the rest of the process. The equipment for initialization is expensive and the added initialization step also increases processing costs. In addition, the reflectivity of the media and the erasing speed of the recording material frequently are not finalized by the initializing step. It can take several read/write cycles for these properties to stabilize. This is highly undesirable, especially since most recording schemes today use pulse-width-modulation. Recording using pulse-width modulation increases the recording density but it also increases the sensitivity to small variations in mark size and shape. It is therefore highly desirable to design a recording element that does not require the initialization step.
Japanese Patent Application No. Hei 7[1995]-47822 discloses a means to eliminate the initialization step for chalcopyrite-type recording materials. According to the disclosure, the Inxe2x80x94Agxe2x80x94Texe2x80x94Sb type recording layer is formed by two separate steps, by first sputtering the Agxe2x80x94Te material and then the Sbxe2x80x94In material. The Agxe2x80x94Te material is said to crystallize even at room temperature thereby giving the high reflectivity required for focusing and tracking. As pointed out by the same inventors in a later Japanese Patent Application JPA10-226173, however, such a procedure produces a recording element in which the reflectivity does not stabilize until the re-written region reaches the entire surface. Furthermore, the re-crystallization rate (the erasure rate) does not stabilize until the element has been rewritten several times. The variation of reflectivity and erasure rate during the first few cycles of re-writing creates high error rates especially if mark edge recording is used.
Japanese Patent Application JPA10-226173 discloses an optical recording element wherein the recording layer is constructed of at least a Sb thin-film containing 95% or more Sb and a reactive thin-film in contact with the Sb thin-film. The Sb thin-film and the reactive thin-film react to form the phase-change material. The Sb thin-film is crystalline and could provide the reflectivity for focusing and tracking during recording. Before the recording element could be used, however, a blending process needs to be carried out. The blending is to mix the Sb thin-film with the reactive thin-film to create the phase-change thin-film. The blending process is similar to the conventional initialization process and a laser light is used to heat up the material. The conventional initialization process crystallizes the recording layer and increases the reflectivity of the recording element. The blending step, on the other hand, results in the dispersion of an amorphous phase in the crystalline Sb phase. The reflectivity decreases, although it is higher than that of the recorded marks. For the Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te based material system, the disclosed media with the blending process was said to have improved stability of reflectivity and erase rate during overwrite cycles and improved durability of media for overwrite cycling. From a media manufacturing point of view, however, JPA10-226173, only replaces the initialization step with the blending step, the cost saving is minimum.
It is the object of the present invention to provide a rewriteable optical recording element of the phase-change type which can be used without using the conventional laser initialization step or the blending step.
It is the further object of the present invention to provide a recording element and a recording method to provide improved stability of the reflectivity and erasure rate from the first recording as well an improved cycling durability.
These objects are achieved by a rewriteable phase-change optical recording element comprising:
(a) a substrate; and
(b) a recording layer over the substrate having at least two sub-layers of different chemical compositions, the compositions and thickness of the sub-layers being selected so that when subject to laser light during a writing process the compositions can mix together to form a high reflectivity mixed crystalline region without using any prior initialization or mixing processes.
It is a feature of the present invention that by proper selection of the recording layer compositions a desired recording performance can be achieved for different applications.
Another feature of the invention is that by providing the recording layer as two sub-layers of different compositions; the thickness ratio of the two sub-layers can be selected such that when these sub-layers are mixed, the optimum erasable phase-change composition results. The sub-layers can be further selected such that at least one of the sub-layers is in the crystalline state either as deposited or after a simple, mild heat treatment. When only one of the two sub-layers is crystalline, this layer is deposited first nearer to the substrate. The proper recording layer composition is selected such that it can be melted by the erase power during the recording process and be crystallized after it solidifies from the melt. The combined thickness of the sub-layers is selected to provide proper optical and thermal tuning of the multi-layer media package.
A rewriteable phase-change optical recording element in accordance with the present invention can use a high erasable power to ensure that the recording layer is melted during the erasing cycle of the recording process.
A preferred embodiment uses a recording layer composition represented by the formula Sb1-x-mxTexInmx wherein by 0.63xe2x89xa7mxe2x89xa70.15, and 0.35xe2x89xa7xxe2x89xa70.28, The first sub-layer consists essentially of Sb, the second sub-layer consists essentially of TeInm wherein 0.63xe2x89xa7mxe2x89xa70.15.